1. Field of the Invention
This invention relates to a high-speed, long-haul communication transmission circuit by an optical fiber, and more particularly to an optical communication transmission system which is expected to be developed as a communication system for a transmission network for advanced information service and which can transmit a large amount of information with a high degree of quality over a long distance.
2. Description of the Related Art
An optical communication transmission system makes use of the broad band feasibility of light to permit high-speed, very high-capacity, high-quality communications which cannot be realized readily with conventional communications using the microwave band or the millimeter wave band. For example, the following reports have been provided with regard to elements for use with communication of, for example, 10 Gbit/s:
by T. Suzaki et al., xe2x80x9c10-Gbit/s Optical Transmitter Model with Multiquantum Well DFB LD and Doped-channel Hetero-MISFET Driver IC,xe2x80x9d 1990 Optical Fiber Communication Conference, Technical Digest TUI2, and
by T. Suzaki et al., xe2x80x9cTen-Gbit/s Optical Transmitter Module Using Modulator Driver IC and Semiconductor Modulator,xe2x80x9d Optical Fiber Communication Conference 1992, Technical digest TUI6.
An optical communication transmission system of the optical amplifier lumped repeater system which uses erbium-doped optical fiber amplifiers will be described with reference to FIG. 1.
An optical transmitter 3 modulates optical power outputted from a semiconductor laser source 1 by intensity modulation by an external modulator 2 of lithium niobate LiNbO3 which is driven by a signal of 10 Gbit/s outputted from a modulation signal source 5 and outputs the modulated optical power to an optical power amplifier 11. The optical power amplifier 11 consists of an erbium-doped optical fiber amplifier and amplifies a signal light level and outputs the amplified optical signal to a first optical fiber 101 for a transmission line of an optical amplifier lumped repeater system. In this instance, when the signal light level exceeds 10 dBm, in order to avoid the influence of Brillouin scattering in the transmission fiber, the line width of the semiconductor laser is expanded in advance using the well-known technique of direct FM modulation of the semiconductor laser or a like technique. After passing the optical fiber 101, the optical signal is amplified again by a direct optical amplifier repeater 12 which consists of an erbium-doped optical fiber amplifier and is then outputted to a second stage optical fiber 111 for transmission. The signal light inputted into the transmission line at the second stage is amplified by a second stage optical amplifier repeater 13 and outputted to a third transmission line 121. The signal light is thereafter processed in a similar manner and transmitted finally to a last transmission line 191. In an optical receiver 53 on the reception side, the optical signal is amplified by an optical preamplifier 21 and converted into an electric signal using a PIN photodiode 51, which is a photoelectric transducer. The electric signal, and consequently, the signal of 10 Gbit/s transmitted from the modulation signal source 5, is then reproduced by an equalizer amplifier regeneration circuit 52.
In the high-speed, high-capacity communication system described above, however, it is known that waveform distortion after transmission due to such causes as chromatic dispersion of the optical fibers strongly degrades the transmission characteristic through a very long distance transmission.
Therefore, the following countermeasures are conventionally taken:
First, as a countermeasure to chromatic dispersion of an optical fiber, which is conventionally considered to be the most significant cause of degradation of the transmission characteristic, a transmission line is constructed using an optical fiber which has no chromatic dispersion in the waveband of the light source of the optical transmitter. In other words, the optical fiber employed has zero chromatic dispersion.
For example, as a communication system for a long-distance submarine cable, transmission systems wherein the dispersion value of an optical fiber for transmission is reduced substantially to zero have been proposed by:
N. S. Bergano et al., xe2x80x9c9000 km, 5 Gbit/s NRZ Transmission Experiment Using 274 Erbium-doped Fiber-Amplifiers,xe2x80x9d Technical Digest of Topical Meeting on Optical Amplifiers and Their Applications, Santa Fe, Jun. 24-26, 1992, postdeadline paper PD11, and
T. Imai et al., xe2x80x9cOver 10,000 km Straight Line Transmission System Experiment at 2.5 Gbit/s Using In-Line Optical Amplifiers,xe2x80x9d Technical Digest of Topical Meeting on Optical Amplifiers and their Applications, Santa Fe, Jun. 24-26, 1992, postdeadline paper, PDI2.
In an actual transmission line, however, the requirement for zero chromatic dispersion cannot be fully satisfied over the entire length of the optical fiber, and very small level of chromatic dispersion exists. In order to suppress the influence of the very small dispersion, several techniques for compensating for the chromatic dispersion in the transmitter side and the receiver side have been proposed, for example, in Japanese Patent Laid-open No. 1987-65529 and Japanese Patent Laid-open No. 1987-65530, and by:
A. H. Gnauck et al., xe2x80x9cOptical Equalization of Fiber Chromatic Dispersion in a 5 Gbit/s Transmission System,xe2x80x9d Optical Communication Conference, San Francisco, Jan. 22-26, 1990, postdeadline paper PD7, and
N. Henmi et al., xe2x80x9cA Novel Dispersion Compensation Technique for Multigiga-bit Transmission with Normal Optical Fiber at 1.5 Micron Wavelength,xe2x80x9d Optical Fiber Communication Conference 1990, postdeadline paper PD8.
Further, in a coherent communication system, such techniques as equalizing an electric signal in the receiver side by using a delay equalizer at the stage of an intermediate frequency of the electric signal have been reported by:
K. Iwashita et al., xe2x80x9cChromatic Dispersion Compensation in Coherent Optical Communicationsxe2x80x9d, IEEE, Journal of Lightwave Technology, Vol. 8, NO. 3, March 1990, pp. 367-375.
It is known that the causes for degradation of the transmission characteristic of an optical amplifier lumped repeater system include, in addition to wavelength dispersion of the optical fiber described above, a noise accumulation effect caused by spontaneous emission light and a noise increase effect caused by a non-linear effect in the optical fiber through multistage optical amplifier repeaters. In order to decrease the influence of the accumulation effect of noise of spontaneous emission light, the outputs of the optical amplifier repeaters must be set high. On the other hand, in order to suppress the non-linear effect in the optical fiber, the outputs of the optical amplifier repeaters must necessarily be set low. Due to these two contradictory requirements, it is conventionally difficult to simultaneously control both the noise accumulation effect and the non-linear effect. Therefore, in order to obtain a very long-haul transmission system or achieve an increase of the repeating distance, it is necessary to increase the repeater output while decreasing the non-linear effect in the optical fiber.
However, little is known of the non-linear effect in an optical fiber, and the causes of degradation have not been specifically identified as yet.
It is believed that a self-phase modulation effect is a major factor in the non-linear effect in an optical fiber. However, as recently reported by S. Saito et al. [xe2x80x9c2.5 Gbit/s, 80-100 km Spaced In-line Amplifier Transmission Experiments Over 2,500-4,500 km,xe2x80x9d Technical Digest of European Conference on Optical Communication 1991, postdeadline paper 3], in addition to the self-phase modulation effect, noise is increased by the influence of a 4 wave-mixing effect between signal light and spontaneous emission light outputted from the optical amplifier, resulting in the degradation of the transmission characteristic.
Further, in addition to the self phase modulation effect, a noise increase believed to arise from a non-linear effect in an optical fiber for each section of a multistage optical amplifier lumped repeater system was discovered in experiments conducted by the inventors of the present application which will be hereinafter described.
It has been made clear that those noise-increasing effects, other than the self-phase modulation effect, increase with the increase of the signal power and the increase of the transmission distance, and noise is produced over the full length of the transmission line, resulting in a greater spectrum spread and a greater degradation of the signal-to-noise ratio than the self-phase modulation effect. Accordingly, it has become clear that the transmission limit is restricted by the non-linear effect in the optical fiber.
It has become apparent through experiments that the non-liner effect in the optical fiber occurs when the transmission light power is high but is deterred when the optical fiber for transmission does not have a zero dispersion wavelength at the wavelength of the optical signal. Therefore, if an optical fiber which does not have a zero dispersion wavelength at the wavelength of the optical signal is employed as the optical fiber for transmission, the non-linear effect in the optical fiber can be suppressed even when the transmission light power is high.
It is an object of the present invention to provide an optical communication transmission system including an optical amplifier lumped repeater system wherein very high-speed, high-capacity and long-haul optical communications can be realized with a high degree of quality.
In order to attain the object described above, an optical communication transmission system of the present invention includes transmission optical fiber means having a zero dispersion wavelength of a value different from the transmission wavelength of the optical transmitter means with at least two connections between the optical transmitter means and the optical receiver means, and dispersion compensation means for making the sum total of wavelength dispersion substantially equal to zero when the sections are arranged in cascade connection.
In an embodiment of the present invention, the dispersion compensation means is included in each of the sections of the transmission optical fiber means or in the optical transmitter means or the optical receiver means. Further, an optical signal is modulated by the optical transmitter means and received in a coherent system by the optical receiver means, and the influence of wavelength dispersion upon the optical signal over the entire transmission line is compensated by the electric dispersion equalization means. The type of modulation by the optical transmission may be optical frequency modulation, phase modulation or polarization modulation.
Further, the present invention can be applied readily to a conventional system by installing a dispersion compensation optical fiber for a transmission optical fiber, which is conventionally provided on the outside, inside an optical repeater and replacing the optical repeater. Alternatively, it is possible to install a small dispersion compensator such as a grating pair in the apparatus in place of the dispersion compensation optical fiber.
In summary, according to the present invention, in order to suppress the non-linear effect in an optical fiber, the zero dispersion wavelength of the transmission optical fibers, which is conventionally made to coincide with the transmission wavelength, is shifted from the transmission wavelength for each section. By virtue of this means, the present invention has the advantage that the transmission optical power of an optical amplifier lumped repeater system can be increased so as to improve the transmission characteristic, and consequently, a very high-speed, very long-haul optical communication transmission system can be realized readily.
The above and other objects, features, and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate the examples of the preferred embodiments of the present invention.